The whole reason for the selfsufficientish website was to offer a place where anyone can ask, HOW DO I...? So who knows why it has taken us so long to have a HOW DO I? section, but here it is. So if you want to know how to do anything selfsufficientish then here is the place to ask.

So the school polytunnel is progressing at a fabulous pace [wish I had one ! ] beds are made, herb bed in and planted, lesson plans being written ....

But the thing is green on the roof ! Given it is 6.5m span, 9m long and 2.8m high it will be a significant job to clean it. I have purchased Algon, but have also been told the school doesnt have a sprayer suitable for the job. Access is looking challenging at least.

Argh, help !

I have read about the sheet flossing technique, and 'could' do it using and old climbing rope, but would rather just spray something on and be done. Any suggestions ? please ?

get a ladder and tie wrap a piece of wood across it that will rest on the hoops either side, wet the tunnel with water and get a soft broom, you will be able to climb up the side and use the soft broom to remove the algae, you can always put some auto wash or washing up liquid into a bucket of water and dip the broom in that to remove stubborn stains

There is exactly what you need, although I've never used it myself so can't comment on how good it is (or how nasty)
It's called Armillatox, and you spray it on and that's it. The next time it rains it gets washed off, I imagine.

I use ordinary household liquid cleaner and a telescopic floor mop thingy, but I don't get algae on the very top, just the sides.